1. Field of the Invention
The present invention relates generally to spark plugs for internal combustion engines and more particularly to a spark plug having two electrodes and an annular threaded portion for engaging the engine with the spark plug seat located between the spark gap and the threaded portion.
2. Description of Prior Art
Traditional spark plug construction includes an annular metal casing having threads near one end and a ceramic insulator extending from the threaded end through the metal casing and beyond the opposite end. A central electrode is exposed near the threaded end and is electrically connected through the insulator interior to a terminal which extends from the opposite insulator end to which a spark plug wire attaches. An "L" shaped ground electrode extends from one edge of the threaded end of the metal casing into axial alignment with the central electrode to define a spark gap therebetween. The force applied to seal the spark plug in the head is the result of torque transmitted by the threaded metal casing, hence, the threaded portion of the metal casing must be sturdy and of substantial size. A portion of the metal casing is formed to be engaged by a socket tool to provide torque to the threaded portion. The threaded portion is located away from the portion which is engaged by the socket tool.
To facilitate the controlled and efficient exhaust of gases from a combustion chamber, the valves are sometimes increased in size. This may necessitate a decrease in the size of the spark plug, a reduction in the size and sturdiness of the threaded metal casing end, and, in particular, a decrease in the inside diameter of the metal bore of the spark plug and a decrease in the combustion chamber wall area available to threadedly receive the spark plug.
The decrease in the inside diameter of the metal bore of the spark plug reduces the ability of the spark plug to resist carbon build up and similar deposits reducing ignition efficiency. In my U.S. Pat. No. 5,091,672 I teach a spark plug which reduces the deleterious effect of reducing the spark plug size by providing a spark plug having an insulator with a cylindrical body that surrounds a central electrode. The cylindrical body has a first diameter section separated from a second diameter section by a shoulder. A sleeve that surrounds the second diameter has an integral base that is positioned a fixed distance from the tip to the center electrode by the engagement of a flange on the sleeve with a shoulder on the cylindrical body. A radial tab that extends from the sleeve aligns with a slot in the head to establish uniform positioning of the ground electrode. A separate end or retainer nut surrounds the first diameter and engages the flange to locate and position the spark plug within the combustion chamber.